I am working on some forms that include image upload. There are standard two forms for adding and all forms connected with it. It will look like this:

I have an asset class that is polymorphic for other classes (e.g. locations, elements). The problem is that items can be downloaded or updated. For items and locations, I have the following:
accepts_nested_attributes_for :assets, :allow_destroy => true, :reject_if => lambda { |a| a[:asset].blank? }
but it seems to reject if there is no file uploaded. This, in essence, is what we want if it is a new file, but there are cases when we simply update the description with the resource identifier. Above: reject_if will reject this script. How can I make an exception to update this other type of information?
THX